Shock absorber for tensioned filaments



Nov. 24, 1942. H. K,y ISHLER 2,303,278

SHOCK AB'SORBER FOR TENSIONED FILAMENTS `Filed -Jan. 14. 1942 Tiff- VPatented Nov. 24, 1942 UNITED STATES SHOCK ABSORBER FORTENSIONED FILAMENT S Harry Kennethlshler, Emporium, Pa., assignor to of MassachusettsSylvania Electric Products Inc., a corporation Application January 14,1942, Serial No. 426,696`

(ci. 25o-27.5)

15 Claims.

This invention refers to thermionic electron discharge devices, and inparticular to electron tubes with heatable filaments.

Because of the expansion of the filament during heating, it is necessaryto provide a spring:

to take up the slack that would otherwise develop if the filament weremounted at its ends on supports whose distance remains constant.

When the filament is heated during operation,

because of its elasticity and because of the tendetermined by itstension, thickness and mass 1.

according to well established mechanical laws. Such a member will beexcited by any kind of shock, and the filament will continue oscillatingat its natural frequency until its vibrating energy has been transferredinto heat by dampingl Under the normal operating conditions of mostelectron tubes, the damping of the filament is exceedingly low. In thefirst place, there is no air damping in the vacuum, and when certainconditions are fulfilled, the viscosity of the lilament in itself islow. Thus frictional forces are small, and a sustained mechanicalvibration at a natural frequency occurs, which results in the well-knownphenomenon called microphonism, aggravated by the periodic change ofplate current caused by the mechanical filament vibration.

Two methods are known for suppressing the microphonism of amplifiertubes using filamentary cathodes. The first entails the use of shockabsorbers on the sockets and bases of the tubes, :i

yfilament which would execute the largest amplitudes during mechanicalvibration.

The'method according to the invention provides a shock absorber, whichdifferentiates from the known shock absorbers in that it is appliedwithin the tube envelope. It is preferably applied in addition to one oranother of the known lilament dampers or displacers, and, if desired, in

addition to other known shock-absorbing devices applied outside thetubes.

vide a shock absorberinside a thermionic tube.

with a iilamentary electrode,A which absorber, pre1-l vents anymechanical shockimparted to thetube, from being transferred to the.iiiamentary` elecf. trede.

Another object ofthe invention is to provide, means for eliminating thedevelopmentv of. stand`l ing waves (ofthe, kind knownv as elastictrans;-` verse displacement waves) on a filamentary elec"-4 trode, dueto a mechanical displacementimparted to the electrode by4 electrical ormechanical stimuli.

A feature ofthe invention refers to thereduc-y tion of microphonism inthermionic ani'pliiersf.y

Another feature ofItheinvention refers to an elastic filament suspensioninA an electron ldise' charge tube, which is frictionally-coup'led to anauxiliary absorbing member.

It is another main object of; the invention to provide means whichinsurea frictional damping of transverse filament oscillations forveachand every plane of polarization in which an incipient oscillationmay have been excited on any points between the active ends of thefilament. v'It is, generally speaking, the'object of the inf vention toeliminate all resonance properties commonlyl associated withelasticstrings, in 'par ticular those resonance properties whichacterize the natural frequencyv in termsy of the velocity of propagationof a transverse vibrat on and along the length of the string. In'this'respect the invention differs intrinsically from all previously knownmethods forthev reduction of microphonism. The latter are predicatedupon one or the other only of two general prin-- ciples. The first isv ashock absorption, which prevents all mechanical transfer of shocksftpthe filament, the second consists in the applica,- tion of a frictionalforce near or at a point of the filament atv which the amplitude of thenatural frequency of the filament isY expected to be a maximum. Bothmeans are onlypartly effective. The first, because a shock may beimparted to the filament by electrical means, e. g., acondenserdischarge; the second, because the friction at the center of thefilament r'merely re-v duces the dying out of the vibration. Thisfrictional force must, in practice always be kept low.

There are two practical limitations for the application of an effectivedamping near .the center of the hot filament. One is the lowering of thefilament temperature resulting from contact with a heat conductingmember. This contact must, therefore, be reduced to a very short It is aprincipal object of the invention to pro- 55 length 0f the,V lment. S0.$11342 @he d illllgilig f dicate rigid end supports.

member will not substantially interfere with the thermionic emissionover an appreciable length of the filament. Secondly, the pressureapplied to the lament at the damping point, on the other and, must notexceed a certain value. If it does, the member will not act as a damperbut will divide the lament in two lengths, whose ends are rigidlyterminated, which terminations are characterized by perfect reflectionof waves at the point of contact.

The idea of the invention may best be understood by the analogy with anelectric Wave transmission line which is terminated by a load whosevalue is equal to its own characteristic impedance. The new shockabsorber represents exactly this sort of a termination of the string atleast at one of its ends.

The adjustment of the friction at one or both filament supports is,therefore, determinedkby the mass andthe elastic characteristics of thefilament under operational conditions. If the frictional force at anendsupport is made equal to the wave resistance of the lament fortransverse elastic waves, standing waves cannot develop, because thewaves are not reflected at the end support. y

The tension acts in'the direction of the axis of the filament, while thefreedom of motion for 4transversal Vibrations is, of course,perpendicular to'this direction.

For a better understanding of the invention, which involves the theoryof the propagation of transversal waves on a string terminated by rigidsupports,A and by ilexible and frictional supports, reference vmay behad to Figs. '7, 8 and 9, which give a' semi-perspective View of a threedimensional rectangular coordinate system .'c, y, z, in which the restposition of a string is indicated in the z-axis by letters AFCDEGB. Aand B in- Any deformation impressed on the string by external forces ata given instant will be propagated in -both directions, -i-e and 2, withvelocity +o, and the shape of this deformation will be reflected backand fforth indefinitely by the rigid end supports at both ends, thusforming tWo sets of travelling distortions'moving along the string inopposite directions and proper phase relations, unless damping forcesare present. Let the original deformation impressed by external forcesat a certain instant have the form indicated in Fig. 7 by the curvesCDE.

The deflection of point C to C is wholly in the "a: direction, that ofpoints E to E' wholly in the 'y' direction. The deflection ofintermediate points has both :x: and y components, as e. g. D lto D inFig. 7. A wave of this character is called unpolarized, which means thatthere is no preferred direction of the deformation in a single fixedplane going through the filament axis. j In accordance with physicallaws, a wave of 4this type can always -be decomposed into two componentWaves each of which is polarized in one of two planes at right angles toeach other. 'The component of the wave polarized in the :rzplane isindicated in Fig. 8, the other component -polarized in the 'yz-plane isindicated in Fig. 9. If we apply now a damping member say in the cdirection, the @my component that win be damped out is the polarizationcomponent of the original wave in the :rz-plane shown in Fig. 8. 'Theother component, polarized in the 11a-plane not damped out, but will goon being reflected 4at the end supports indenitely. l' -f In ordervtoestablish a damping of this second component, it is necessary to add asecond damping member which offers friction in the y-direction, i. e. ina. direction at right angles to the former damping member.

It is a further fact that the wave will not be reflected at all, if africtional support replaces the rigid end support, and if the frictionalforce acting there has the Value of the wave resistance which isdetermined by tension and mass per unit length of the string.

' On the other hand, if the rigid end support is replaced by an elasticend support, the wave is not damped at all, it will be reflected just asmuch as if the end support were rigid. The only effect of theflexibility of the end support is a change in the natural frequencies ofthe string as compared to the natural frequencies for rigid supports.-As is well known, the lowest natural frequencies are integer multiplesof the fundamental frequency, corresponding to a wave length twice aslong as the length of the string. The flexibility of the end supportsacts as if the length of the string had been increased.

- Therefore in order to insure complete damping of any wave beingpropagated on a string, it is necessary to apply damping members in twodirections at right angles to each other and to the string or filamentaxis, and the value of the frictional force must be equal to the waveresistance which is de rmined by the tension and the mass per unitlength of the string. In applying this knowledge to a filament of anelectron discharge tube, it must also be kept in mind, that thefrictional force can not be replaced by an elastic force acting on theend supports, and that the active frictional end support may be achievedby bending a filament over a substantially rigid guiding member throughan angle which, together with the tension of the filament can be sochosen that a frictional force equal to the wave resistance isestablished.

The invention may be carried out in several ways in practice. Theimportant criterion is, in al1 cases, to provide a free end of thefilament in the direction perpendicular to its own longitudinalextension, and to apply a frictional bearing at this end, which producesa resistance against transverse motion substantially equal to the waveresistance of the filament under operating conditions. Severalembodiments of the invention will now be discussed in connection withthe drawing in which Fig. 1 shows a complete radio tube embodying theinvention. Fig. 2 is a magnied view of part of the electrode mount ofFig. 1 showing one number of reducing the filament microphonism.

Fig. 3 shows a modification of Fig. 2.

Fig. 4 shows another modification of Fig. 2.

Fig. 5 is an enlarged view of one of the mica supports for the filamentshown in Fig. 4.

Fig. 6 is a sectional view of Fig. 2 taken along the line 6-6 thereof.

Referring to Fig. 1 there is shown a typical radio tube embodying theinvention. Merely for .explanatory purposes, the tube is shown as of thetriode type having a filamentary cathode I, a control grid 2, and aplate or anode 3. 'I'he various electrodes are assembled between twomica .spacer discs 4, 5, which'are fastened to a pair of uprights 6, 1,by metal eyelets 8 to form a so- Acalled unitary mount or electrodeassembly. If

desired, a small tubular metal shield 9 may surround the upper exposedend `of* filament l butA in spaced relation thereto. The mountissupported by welding the right-angled ends of uprights 6, 1, to therespective rigid lead-in prongs Il), I I. If desired, the mount mayalso' be supported by engagement' between mica disc 5 and the upper endsof the various lead-in prongs. The prongs Il and I2 for example may -beconnected to opposite ends of the filament. The prong' IS may beconnected to the control grid 2, and prong I s may be connected to theplate 3. It will be understood of course, that any other knownconstruction of electrode assembly may be employed. The electrodeassembly is enclosed within a bulb which is closed ofi at its lower endby'a attened glass header I 8` through which the various rigid lead-inprongs are sealed.

Since the invention is concerned primarily with the manner of supportingthe filament I, reference will now be made to the enlarged view of Fig.2 which omits all Ielectrodes except the filament I which is welded tofilament tabs I5 and I6 at its ends. The tab I5 is welded to a shortmetal strap i'l welded to lead wire I2 which passes through header I8.The other tab I6 is welded to tension spring I9 which is welded at point2l) to support rod A rigid metal bar 2! welded at one end 22 to supportrod 'I rests against tab I5 of filament I and offers friction to anymotion of the filament in the direction of the dotted arrows. Thisfriction should be adjusted, as explained above, to a value equal to thewave resistance of the filament under operating conditions, i. e., itshould be equal to twice the square root of the product of the filamenttension into the mass of the filament per unit length. This adjustmentis easily determined by a few test runs, and can easily be maintainedwithin practical tolerance-limits by appropriate mounting control.

Obviously, the friction acts only in the particular direction shown inFig. l, e. g., in theV plane of the paper. An arbitrary transversaldisplacement of any point of the filament may, however, have any otherdirection perpendicularv to the main extension of they iilament. Inother words, an incipient transversal oscillation of the filament mayhave polarized components in a plane perpendicular to the plane dened bythe axis of the filament and the direction of bar 6, in addition tocomponents in this plane.

The components of the oscillation in a direction perpendicular to thisplane are, however, not damped out by the device according to Fig, 1,and, if necessary, additional means can be provided to offer friction tocomponents of the motion in this direction. Such an'arrangement is shownin Fig. 3. In Fig. 3, filament I is again welded to tabs I'5 and I6. TabI5 is attached to rod '1, tab I5 to tensionr spring I9 which, in turn,is welded to support rod 'I which passes through mica spacer 5. Afriction bar 23 is provided at the lower end of the filament and bearingagainst tab I5, which prevents the reiiection of any components of atransversal displacement wave of the filament at point 24 in thedirection of bar 23: In order to prevent reflection of the components ofthe displacement waves perpendicular to the direction of bar 23, africtional support is provided by slot 25 in top mica 4. As may be seenfrom Fig. 3, the slot allows a free motion of lament in a directionperpendicular to the plane of members I and 23, i. e., perpendicular toline L-L. The friction offered by the edge of the slot in the mica will,however, prevent a reiiection of the components of the transthefilament.

verse displacement wave in the direction of the slot. It will beobserved that the filament is tens-oned so that it is held against'onelongitudinal edge of slot 25.

The combination of bar 23 at thebottom with the frictional end supportoffered by slot 25 in Fig. 3 will, therefore, prevent the setting up ofstanding transversal waves on the filament, independent of the directionofthe plane of polarization of these waves. If desired, rod I may be anintegral continuation of lead-in prong II which passes through headerI8.

It is, of course, immaterial, as far as the invention is concerned, whatparticular kind of frictional end support is applied at the ends of thefilament. Obviously, it is quite possible to provide two frictional bars23 perpendicular to eachother, one at each end of the filament, or toprovide two frictional mica bearings at the two filament ends, disposedat Vright angles to each other.

Fig. 4 shows one embodiment of the invention, in which the twofrictional filament end supports, which act in directions perpendicularto each other, are carried out in a practical way by means of triangularholes in the top and bottom micas 4 and 5. The two triangular holes 26and 27 in mica supports 5 and 4 have rounded corners, as shown moreclearly in enlarged Fig. 5. The radius rounding-off the corners of thetriangular'hole is several times larger than the diameter of'iilament I,and the filament is, therefore, free to move (over short lengths) in thedirection a-rL at the apex of triangular hole 21, substantially in thedirection of the tangent at point 28 of the circle rounding off thecorner at this apex of triangle 2'I. v

The two holes 26 and 2I are so disposed in the micas 5 and 4 that thetangential directions at 28 i. e., a-a and at 29 i. e., b-b, areperpendicular to each other and to the principal axis of The frictionalend bearings,v applied to the filament by this mutual position of theholes in the micas, together with the oblique direction of the tensionapplied to the filament as indicated in Fig. 4, will thus provide aneffective prevention of the reiiection of transversal mechanical wavespolarized in any direction whatever. It will be observed that the holesin the micas are so disposed that point 28is approximately in verticalalignment with point 29.

The tab 30 is oiiset horizontally with respect to hole 2B so that thefilament end 3| extends perpendicular to line b-b.

For practical purposes, it is not necessary that the directions a-a at28 and b-b at 29 are disposed at right angles to each other, because dueto the curvature of the corners, friction is offered not only fortransversal displacements in the direction of the tangent at'28, butover a range of tangents at points in the neighborhood of 28. The sameis, of course, true for the other mica at point 29.

This coverage of friction over a certain circular range thus increaseswith increasing amplitude of the undesired vibration, and the angularrange at each bearing thus increases automatically as needed. Because ofthis automatic increase of the angular range of frictional eiciency, thetangential directions of two cooperating frictional end bearings may bedisposed at an angle different from degrees, if a curvature of thebearing periphery is provided similar to that explained in connectionwith Fig. 5.

Accordingly, it is within the compass of the invention to provide a pairof frictional end supports for a lamentary electrode, of a iinite rangeof angular eiciency, in which the two supports are disposed at an angleto each other, which may be different from 90 degrees. Thus in the caseof very small tubes the dimensions of the tube elements may be so smallthat not enough space is available for threading filamentary cathodesthrough the two micas, if the holes are disposed at right angles to eachother.

While certain specific 'embodiments of the inventive subject matter havebeen illustrated, it will be understood that various changes andmodifications may be made therein without departing from the spirit andscope of the Vinvention.

What I claim is:

1. In combination, a filamentary member, means to maintain said memberunder tension, and means to counteract the development on said member ofstanding Waves of the elastic transverse displacement type when saidmember is subjected to shock excitation, and comprising means to applyfrictional damping to opposite ends of the filament in mutuallyperpendicular directions.

2. In combination, a heatable filament, means to maintain said lamentunder tension. and means to counteract the development of microphonismwhen said lament is subjected to shock, the last-mentioned meansincluding a shock absorber frictionally coupled to said lament adjacentan end thereof, and another shock absorber frictionally coupled to saidlament adjacent the opposite end thereof, said shock absorbers applyingfrictional forces in mutually perpendicular relations with respect tothe length of the filament.

3. In combination, a iilamentary member having an active lengthintermediate the ends thereof, means to maintain said member undertension, and means to damp transverse oscillations of said filament forsubstantially all planes of polarization in which the lament is shockedat any point of its active length, and including friction means adjacentopposite ends of the lament for applying frictional forces theretoacting in mutually perpendicular relations with respect to the length ofthe lament.

4. In combination, a heatable lament, means to maintain said lamentunder tension, and means adjacent the opposite end sections of thefilament to provide frictional forces at said opposite end sections todamp transverse filament oscillation in mutually perpendicular planes.

5. The combination according to claim 2 in which said shock absorberseach comprises a substantially rigid member extending substantiallytransverse to the length of the filament,

and frictionally coupled thereto to provide frictional forces transverseto the length of the iilament.

6. The combination according to claim 4 in which said damping meanscomprises a pair of members extending substantially perpendicular to thefilament length and each coupled frictionally thereto.

` 7. In combination a heatable lamerit, means to maintain the filamentunder tension, means frictionally coupled to the filament adjacent oneend to damp vibrations in one direction perpendicular to the filamentaxis Without interfering With the free expansion and contraction of thefilament, and means frictionally coupled to the filament adjacent theopposite end, to damp out vibrations perpendicular to both the filamentaxis and to said one direction.

8. The combination according to claim 'I in which the damping meanscomprises a pair of members having substantially rigid edges extendingtransversely to each other and frictional- 1y engaging the filament sothat the line of action of the frictional forces at one end of thefilament is transverse to the line of action of the frictional forces atthe other end of the filament.

9. The combination according to claim "I in which the damping meanscomprises a pair of members having substantially rigid edges extendingtransversely to each other and frictionally engaging the lament, andsaid tension means exerts a component of force holding said filamentagainst said edge.

10. The combination according to claim 7 in which the damping meanscomprises a pair of rigid rods extending transversely to the lament butin different planes and frictionally engaging opposite ends of thefilament.

11. The combination according to claim 2 in which the frictionalcoupling is adjusted to equal approximately 2\/t.m. Where t is thefilament tension and m is the lament mass per unit length.

12. The combination according to claim '7 in which the damping meanscomprises a substantially rigid rod frictionally coupled to the filamentadjacent one end, and a disc having a slot adjacent the opposite end ofthe lament and through which the filament passes, said slot extending ina diierent direction from said rod.

13. The combination according to claim '7 in which the damping meanscomprises a pair of discs mounted adjacent the end sections of thelament and each having a perforation through which the filament passes,and said tensioning means maintains the lament against an edge of eachperforation.

14. 'Ihe combination according to claim 7 in which the damping meanscomprises a pair of discs mounted adjacent the end sections of thelament and each having a perforation with a rounded corner and throughwhich the filament passes, and said tensioning means maintains thelament against the edge of said corner.

15. The combination according to claim 7 in which the damping meanscomprises a pair of discs mounted adjacent the end sections of theiilament and each having a rounded notch, the opening angle of one notchbeing substantially transverse to the opening angle of the other notch,and said tensioning means maintains the filament against the roundededge of each notch.

HARRY KENNETH ISHLER.

